Inelastic tunneling

Inelastic tunneling

xvi CONFERENCE ON TUNNELLING effective density of states which is the spectral function of the Green’s functions with both variables in the barrier...

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CONFERENCE ON TUNNELLING

effective density of states which is the spectral function of the Green’s functions with both variables in the barrier. The effect of impurities is in the decrease of the effective density of states. The effective density of states is determined by the scattering amplitude of the conduction electrons scattered on the impurities, We discuss two models of the scattering mechanism: a) Abrikosov-Suhl resonance with resonance energy E 0# 0, b) Kondo boundstate, The characteristics corresponding to different signs of the exchange coupling are the following. Ferromagnetic coupling: Conductance maximum at zero bias (relative amplitude 20%). Antiferromagnetic coupling: mum; 1) large 2) relative small relative amplitude, amplitude, resistivity a) in maxithe case of Abrikosov-Suhl resonance, conductance maximum at zero bias on a background with resistivity maximum, b) Kondo boundstate, resistivity maximum. We may conclude that the experimental distinguish between the two different scattering amplitudes would be possible only in those cases in which the relative amplitude of the zero bias anomalies is not larger than 1-2, because the large change in the effective density of states has a drastic effect in the scattering amplitude. ELECTRON TUNNELING AS A PROB~EOF BARRIER EXCITATION, D. 3, Scalapino, U. of Pennsylvania, Philadelphia, Pa,, USA, Electron tunneling has provided detailed information about the elementary excitations and their interactions in superconductors. Recently, interest has shifted towards using electron tunneling to probe the excitations of the insulating region and the metal-insulator surface of tunnel junctions. Results of a calculation have been reported which show that the structure observed by Jaklevic and Lambe is proportional to the dipole spectral weight of a molecule in the barrier. Here we report more details of this work and extend it to the Josephson pair current, The problem can be separated into two parts: (A) a determination of the dispersion relation of the barrier excitations and the strength of their coupling to the tunneling electrons; and

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(B) given the Harniltonian describing the dynamics of (A), determine the structure of the I-V characteristics. It is possible to give

a formal solution to part (B) whtch relates the I-V characteristic to certain averages of the spectral weight of the barrier excitations. It is this second aspect of the problem which will be discussed here. We believe that this formal approach is useful since it provides a framework for interpreting the I-V characteristics so that the experimental results can be directly brought to bear on the more difficult part of the problem posed by (A). Using a model Hamiltonian to describe the tunnel junction, a Kubo relation for the tunneling current is obtained. The structure in the quasi-particle I-V characteristics associated with single excitation,production is determined. 21/dV’ provides detailed inWe formation show that on the d spectral weight function of the excitations folded into the form factor for excitation production. The effect of barrier excitations on the magnitude of the critical Josephson current carried by a superconducting junction is evaluated. The possibilities of observing this effect by studying the Josephson critical current in the presence of radiation will be discussed. INELASTIC TUNNELING, John Lambe and R. C. Jaklevic, Scientific Laboratory, Ford Motor Company, Dearborn, Michigan, USA. Experiments have been carried out relating to the process of inelastic tunneling between metal films. We have also carried out an analysis in terms of the likely mechanisms of such effects. The experiments have been of the type already reported whereby various molecular additions are put into the barrier of metal-metal oxide-metal junctions. In general, the spectra are in good agreement with what one would anticipate from the various materials inserted. It is worth noting that harmonics of the 0-H stretching vibrations have been seen at 0. 94 volts which indicates that the spectral range can be extended into the visible region. Studies have been made of the spectral linewidth as a function of tempera-

ture from l°K up to 300°K. For an inelastic process involving excitations of narrow natural linewidth one would predict an observed linewidth of 5. 4 kT due to thermal smearing of the Fermi distribution, This is in good agreement with the linewidths observed at higher temperatures where thermal broadening

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CONFERENCE ON TUNNELLING

should dominate. The use of superconductors does enhance spectral resolution as would also be predicted for inelastic processes. In terms of mechanisms one can show that coupling to IR active modes should occur as discussed by Scalapino and Marcus. In addition, we find theoretically that there also exists a coupling to Raman active modes which could be of comparable strength. No experiments bearing directlyon the relative strengths of these processes have been made as yet since the

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molecules added have not been of the proper symmetry to make such a differentiation. Spectra taken in the 0-50 my range in normal Al-Al oxide-normal Pb or Al junctions have revealed structure which appears to correspond to phonons of these metals. The structure is such~that interpretation in terms of an inelastic tunneling process is possible. The tunneling electron-metal phonon coupling mechanism is not yet established.

Calendar of Solid State Events Items for inclusion in the Calendar should state the title, date, location and sponsors of the meeting, and also the name and address of the person to whom inquiries should be directed. Announcements for publication should be mailed to “Editor, Solid State Communications,” at Pergamon Press, 44-01 21st Street, Long Island City, N.Y. 11101. 1968 17-21 June (tentative) Symposium on the Structure and Chemistry of Solid Surfaces, Berkeley, California.

23-27 September An International Conference on Vacancies and Interstitials in Metals, Kernforschungsanlage J(ilich (Federal Republic of Germany).

Sponsored by the Inorganic Materials

Research Division of the Lawrence Radiation Laboratory, the University of California at Berkeley and the College of Chemistry. Information’ C. V. Peterson Arrangements Chairman Fourth International Materials Symposium Lawrence Radiation Lab. Building 62, Room 205 Berkeley, California 94702, U. S. A. 15-19 July 2nd International Conference on Crystal Growth, University of Birmingham, England. Information’ Professor A, D. McQuillan, Department of Physical Metallurgy and Materials Science, University of Birmingham, Birmingham 15, England.

Experimental and theoretical work which elucidates the generation and properties of vacancies and interstitials (including foreign interstitial atoms to the extent that the work contributes to the general understanding of the field) small agglomerates of these. Emphasis will and be on work that gives information on the fundamental physical properties of the defects (atomistic, electronic, and magnetic structure, formation energy, vibrational behaviour etc.), on their mobility, their interactions and their reactions with each other or with impurities and dislocations (e. g. recombination, clustering, trapping, pInning). The results of less widely used experimental

techniques will be welcomed, e. g. diffuse X-ray and neutron scattering, anomalous transmission, field ion-microscopy, thermoelectric, galvanomagnetic, mechanical and magnetic relaxation techniques etc. Information: Dr. Werner Schilling Kernforschungsanlage Jtillch Institut ftlr Festk~irper-und Neutronenphysik 5170 Jtillch, Postfach 365, Germany.